TFTs (thin-film transistors) are used as switching elements in active-matrix-driven panels such as LCDs (liquid crystal displays). The most common method for forming the semiconductor layer of TFTs is a method of patterning, by photolithography, an amorphous silicon film formed by CVD. Whereas the mobility of amorphous silicon is about 0.5 cm2/V·s, in recent years TFTs using oxide semiconductors whose mobility values are as high as 10 cm2/V·s have been being developed.
Semiconductor materials commonly used for TFTs, such as amorphous silicon, have as small band gaps as about 1 eV and are not transparent (i.e., have absorptions in the visible range). In contrast, in general, oxide semiconductors have as large band gaps as 3 eV and are transparent (i.e., have no absorptions in the visible range). In JP-A-2007-81362 (corresponding to US2007/0057261A1), for example, these features are utilized to increase the aperture ratio and the resolution of a display.